One known family of porous crystalline materials are zeolitic materials, which are based on the 3-dimensional, four-connected framework structure defined by corner-sharing [TO4] tetrahedra, where T is any tetrahedrally coordinated cation. Among the known materials in this family are silicates that contain a three-dimensional microporous crystal framework structure of [SiO4] corner sharing tetrahedral units, aluminosilicates that contain a three-dimensional microporous crystal framework structure of [SiO4] and [AlO4] corner sharing tetrahedral units, aluminophosphates that contain a three-dimensional microporous crystal framework structure of [AlO4] and [PO4] corner sharing tetrahedral units, and silicoaluminophosphates (SAPOs), in which the framework structure is composed of [SiO4], [AlO4] and [PO4] corner sharing tetrahedral units. Included in the zeolitic family of materials are over 200 different porous framework types, many of which have great commercial value as catalysts and adsorbents.
Zeolitic imidazolate frameworks or ZIFs have properties similar to inorganic zeolitic materials. ZIFs are based on a [M(IM)4] tetrahedral coordination bonding environment in which IM is an imidazolate-type linking moiety and M is a transition metal. These materials are generally referred to as zeolitic imidazolate frameworks or ZIFs since the angle formed by imidazolates (IMs) when bridging transition metals is similar to the ˜145° angle of the Si—O—Si bond in zeolites. ZIF counterparts of a large number of known zeolitic structures have been produced. In addition, porous framework types, hitherto unknown to zeolites, have also been produced. Discussion of this research can be found in, for example, the following publications from Yaghi and his co-workers: “Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks”, Proceedings of the National Academy of Sciences of U.S.A., Vol. 103, 2006, pp. 10186-91, “Zeolite A Imidazolate Frameworks”, Nature Materials, Vol. 6, 2007, pp. 501-6, “High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture”, Science, Vol. 319, 2008, pp. 939-43, “Colossal Cages in Zeolitic Imidazolate Frameworks as Selective Carbon Dioxide Reservoirs”, Nature, Vol. 453, 2008, pp. 207-12, “Control of Pore Size and Functionality in Isoreticular Zeolitic Imidazolate Frameworks and their Carbon Dioxide Selective Capture Properties”. Journal of the American Chemical Society. Vol. 131, 2009, pp. 3875-7, “A Combined Experimental-Computational Investigation of Carbon Dioxide Capture in a Series of Isoreticular Zeolitic Imidazolate Frameworks”, Journal of the American Chemical Society, Vol. 132, 2010, pp. 11006-8, and “Synthesis, Structure, and Carbon Dioxide Capture Properties of Zeolitic Imidazolate Frameworks”, Accounts of Chemical Research, Vol. 43, 2010, pp. 58-67.
Several ZIFs are known to have good thermal and chemical stability, high microporosity, and high internal surface area. ZIFs have therefore created substantial interest for potential use in diffusive and adsorptive separations. In particular, ZIF-7 (in which the imidazolate-type linking moiety is benzimidazole) has been the focus of extensive research efforts, at least partly because the material undergoes an unusual and reversible phase change transition from a narrow-pore to large-pore form both on heating and during adsorption of guest molecules. See, for example, Du, Y.; Wooler, B.; Nines, M.; Kortunov, P.; Paur, C. S.; Zengel, J.; Weston, S. C.; Ravikovitch, P. L J. Am. Chem. Soc. 2015, 137, 13603-13611. This represents a significant opportunity for the potential use of ZIF-7 and related structures in gas separation and storage.
Of all the ZIFs discovered, only a few are known to undergo such displacive transitions from a nearly nonporous to a porous structure upon adsorption of guest molecules. This guest responsive phase change usually results in a step change in the adsorption isotherm. Other examples of ZIF materials exhibiting this property are ZIF-9 and EMM-19. In these materials, the nature of the adsorption is fixed; the pressure in which phase change and subsequent adsorption occurs is an intrinsic property of the material and the guest being adsorbed and is not tunable. The ability to tune these adsorption properties synthetically has been a goal of many researchers. Some CO2 sorbent materials do exhibit this synthetic tunability (see, for example, Mason, J. A.; Oktawiec, J.; Taylor, M. K.; Hudson, M. R.; Rodriguez, J.; Bachman, J. E.; Gonzalez, M. I.; Cervellino, A.; Guagliardi, A.; Brown, C. M.; Llewellyn, P. L.; Masciocchi, N.; Long, J. R. Nature 2015, 527, 357-361). However, it is reliant on the adsorption of a reactive gas such as CO2 and is not a general technique. Other researchers have attempted to use a “mixed-linker approach” to tune the adsorption properties of ZIF-7 (see, for example, Thompson, J. A.; Blad, C. R.; Brunelli, N. A.; Lydon, M. E.; Lively, R. P.; Jones, C. W.; Nair, S. Chem. Mater. 2012, 24, 1930-1936). In the cited work, linkers such as 2-methylimidazole or 2-carboxylimidazole were incorporated into ZIF-7 in an attempt to modulate the adsorption properties of the ZIF-7. However, this approach proves to be difficult because the doping of ZIF-7 with any other imidazole linker is difficult synthetically often resulting in materials with only a small amount of the desired dopant linker actually incorporating into the material.
There is therefore a need for new methods of tuning the composition and adsorption properties of zeolitic imidazolate frameworks, especially those, such as ZIF-7, which exhibit phase change transitions.